1. Field of the Invention
This invention relates to fiber optic pressure sensors, and more particularly to a Bragg grating pressure sensor.
2. Description of the Related Art
Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a pressure sensor, the sensing element may be bonded to the elastic structure with a suitable adhesive.
It is also known that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors which measure static or very slowly changing parameters, the long term stability of the attachment to the structure is extremely important. A major source of such long term sensor instability is a phenomenon known as xe2x80x9ccreepxe2x80x9d, i.e., change in strain on the sensing element with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal.
Certain types of fiber optic sensors for measuring static and/or quasi-static parameters require a highly stable, very low creep attachment of the optical fiber to the elastic structure. Various techniques exist for attaching the fiber to the structure to minimize creep, such as adhesives, bonds, epoxy, cements and/or solders. However, such attachment techniques may exhibit creep and/or hysteresis over time and/or high temperatures.
One example of a fiber optic based sensor is that described in U.S. patent application Ser. No. 08/925,598 (now U.S. Pat. No. 6,016,702), entitled xe2x80x9cHigh Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments,xe2x80x9d to Robert J. Maron, which is incorporated herein by reference in its entirety. In that case, an optical fiber is attached to a compressible bellows at one location along the fiber, and to a rigid structure at a second location along the fiber. A Bragg grating is embedded within the fiber between these two attachment locations with the grating being in tension. As the bellows is compressed due to an external pressure change, the tension on the fiber grating is reduced, which changes the wavelength of light reflected by the grating. If the attachment of the fiber to the structure is not stable, the fiber may move (or creep) relative to the structure it is attached to, and the aforementioned measurement inaccuracies occur.
In another example, an optical fiber Bragg grating pressure sensor where the fiber is secured in tension to a glass bubble by a UV cement is discussed in Xu, M. G., Beiger, H., Dakein, J. P., xe2x80x9cFibre Grating Pressure Sensor With Enhanced Sensitivity Using A Glass-Bubble Housing,xe2x80x9d Electronics Letters, 1996, Vol. 32, pp. 128-129.
However, as discussed hereinbefore, such attachment techniques may exhibit creep and/or hysteresis over time and/or high temperatures, or may be difficult or costly to manufacture.
Objects of the present invention include provision of a fiber optic pressure sensor with minimal creep.
According to the present invention, a pressure sensor comprises an optical sensing element, having at least one pressure reflective element disposed therein along a longitudinal axis of the sensing element, the pressure reflective element having a pressure reflection wavelength; the sensing element being axially strained due to a change in external pressure, the axial strain causing a change in the pressure reflection wavelength, and the change in the pressure reflection wavelength being indicative of the change in pressure; and at least a portion of the sensing element having a transverse cross-section which is contiguous and made of substantially the same material and having an outer transverse dimension of at least 0.3 mm.
According further to the present invention, the sensing element comprises: an optical fiber, having the reflective element embedded therein; and a tube, having the optical fiber and the reflective element encased therein along a longitudinal axis of the tube, the tube being fused to at least a portion of the fiber. According further to the present invention, the sensing element comprises a large diameter optical waveguide having an outer cladding and an inner core disposed therein and an outer waveguide dimension of at least 0.3 mm.
According still further to the present invention, the reflective element is a Bragg grating. According still further to the present invention, the sensing element has a dogbone shape. According still further to the present invention, the sensing element comprises a dogbone shape and comprises an outer tube fused to at least a portion of large sections of the dogbone shape on opposite axial sides of the reflective element.
The present invention provides a fiber grating disposed in an optical sensing element which includes an optical fiber fused to at least a portion of a glass capillary tube (xe2x80x9ctube encased fiber/gratingxe2x80x9d) and/or a large diameter waveguide grating having an optical core and a wide cladding, which is elastically deformable based on applied pressure. The invention substantially eliminates creep and other optical fiber attachment problems. The sensing element may be made of a glass material, such as silica or other glasses. Also, the invention provides sensing with very low hysteresis. The present invention allows forces to be applied axially against the sensor element end-faces thereby allowing for high sensor sensitivity. The present invention also provides improved sensor reliability when used in compression. Also, one or more gratings, fiber lasers, or a plurality of fibers may be disposed in the element.
The grating(s) or laser(s) may be xe2x80x9cencasedxe2x80x9d in the tube by having the tube fused to the fiber on the grating area and/or on opposite axial sides of the grating area adjacent to or a predetermined distance from the grating. The grating(s) or laser(s) may be fused within the tube or partially within or to the outer surface of the tube. Also, one or more waveguides and/or the tube encased fiber/gratings may be axially fused to form the sensing element.
Further, the invention may be used as an individual (single point) sensor or as a plurality of distributed multiplexed (multi-point) sensors. Also, the invention may be a feed-through design or a non-feed-through design. The sensor element may have alternative geometries, e.g., a dogbone shape, that provides enhanced force to wavelength shift sensitivity and is easily scalable for the desired sensitivity.
The invention may be used in harsh environments (high temperature and/or pressure), such as in oil and/or gas wells, engines, combustion chambers, etc. For example, the invention may be an all glass sensor capable of operating at high pressures ( greater than 15 kpsi) and high temperatures ( greater than 150xc2x0 C.). The invention will also work equally well in other applications independent of the type of environment.
The foregoing and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.